1. Field of the Invention
The present invention relates to an apparatus and method for controlling the position of a dispensing head for dispensing material onto a substrate in a dispensing system. More particularly, it relates to an apparatus and method for changing the height or position of a foot used to position the needle of a dispensing head.
2. Discussion of Related Art
There are several types of prior art dispensing systems used for dispensing metered amounts of liquid or paste for a variety of applications. One such application is in the assembly of printed circuit boards and integrated circuit chips. In this application, dispensing systems are used in the process of encapsulating integrated circuits with an encapsulating material and in the process of underfilling flip integrated circuit chips with an encapsulant. Prior art dispensing systems are also used for dispensing dots or balls of liquid epoxy or solder paste onto circuit boards and integrated circuits. The liquid epoxy and solder are used to secure components or connect components, respectively, to a circuit board or to an integrated circuit. The dispensing systems described above include those manufactured and distributed by Speedline Technologies, Inc, the assignee of the present invention, under the name CAMALOT.
The dispensing systems described above are typically used in an electronics manufacturing facility in an automated assembly line with other equipment used in a circuit board or integrated circuit manufacturing process. The other equipment in-line with the dispensing systems may include, for example, pick and place machines, which place components on circuit boards, or reflow ovens that are used to heat materials, such as solder paste, dispensed onto the circuit boards or integrated circuits.
In a typical dispensing system, a pump and dispenser assembly is mounted to a moving assembly for moving the pump and dispenser assembly along three mutually orthogonal axes (x, y, z) using servomotors controlled by a computer system or controller. To dispense a dot of liquid on a circuit board at a desired location, the pump and dispenser assembly is moved along the horizontal x and y axes until it is located over the desired location. The pump and dispenser assembly is then lowered along the vertical z axis until the needle of the pump and dispenser assembly is at an appropriate dispensing height over the board. The pump and dispenser assembly dispenses a dot of liquid, is then raised along the z axis, moved along the x and y axes to a new location, and is lowered along the z axis to dispense the next liquid dot.
Dispensing systems can be characterized generally as footed or non-footed systems. Non-footed systems are adapted for applications that do not require contact between the dispense tip and the substrate during dispensing. In non-footed applications, the dispense tip is positioned and suspended above the substrate by a predetermined amount, and the fluid is dropped onto the substrate from above. The height above the substrate must be carefully programmed and calibrated relative to the substrate surface. In footed applications, the tip is provided with a standoff, or “foot”, designed to contact the substrate as fluid is delivered by the pump through the tip. The programmed dispense height is set below the surface of the substrate so that the foot reliably contacts the substrate, even as the surface of the substrate varies. Such footed systems allow for tip travel, relative to the pump body, such that the entire weight of the pump does not bear down on the substrate. In many instances, it is important that the distance between the end of the nozzle or dispensing needle and the substrate be precisely controlled. One popular method for controlling this gap is the use of a foot adjacent to the needle. The foot is positioned adjacent the needle of the pump and offset along the z axis to provide the proper distance between the needle and the substrate. When the footed needle is lowered toward the substrate, the foot will contact first and will maintain the needle end at a fixed height above the substrate.
Conventional dispensing systems include a replaceable needle assembly which is attached at the output of the pump with a needle nut. The needle assembly includes a base and a needle extending from the base. The base is shaped to fit within needle nut in a single position. Typically, the base is round with a flat surface. The flat surface on the base engages a tooth on the dispenser. During assembly, the needle is turned to engage the tooth to prevent further rotation of the needle as the needle nut is tightened. A replaceable needle assembly allows different size or length needles to be used with a single dispensing system. Also, needles may wear or break and need to be replaced. With a footed system, the needle foot is formed with the needle as an integral part of the needle assembly. Different needle assemblies have feet at different heights. Thus, in order to change the height of the foot, the needle assembly has to be replaced. In order to accommodate different size needles and different height feet, a large inventory of needle assemblies are required.
Since the foot contacts the substrate during dispensing, it will wear more quickly than the needle. Since precision in the height is necessary for proper dispensing, the entire needle assembly must be replaced when the foot becomes worn. This requires more frequent replacement of footed needle assemblies.
Furthermore, since the needle assembly is in a fixed position relative to the needle nut and dispensing system, the position of the foot cannot be changed. Since the foot is caused to contact the substrate, it is imperative that the foot not be placed in a previous deposit of adhesive. To this end, various methods have been developed to prevent “stepping” in a previous deposit. One method includes the use of a needle with an orientation feature that causes the needle to be mounted in a specific orientation. With the needle orientation fixed within the dispensing system, the position of the needle foot can be determined. However, different applications may require different positions. Thus, footed needles are available with the foot at different fixed positions—0°, 45°, 90°, etc.—relative to the needle. If the dispensing pattern and sequence are known, an appropriate needle may be selected such that the foot is caused to march ahead of the needle position, thereby avoiding its placement in a previous deposit. This approach has the disadvantage that a considerable inventory of various needle sizes, heights, and orientations may be required to support a variety of applications. Also, the pattern has to be carefully designed to accommodate the specific needle and foot.
U.S. Pat. No. 6,299,078 discloses an adapter plate for a footed needle so that it can be mounted in one of eight positions. The adapter plate eliminates the need for multiple needles. The foot may be positioned differently for different application. However, this system still requires careful design of the pattern and sequence to accommodate the selected position. Sometimes, the design of the substrate complicates selection of a position of the foot. Many substrates have features that deviate from a flat plane. For example, the thickness of the traces on a printed circuit board extend above the level of the remainder of the substrate. In such circumstances, if the foot contacts an elevated area, a different needle stand-off height may result. Therefore, a need exists for a dispensing system in which the orientation of the needle and foot can be adjusted as required by the dispensing process. Additionally, if an integral footed needle were rotated to change the orientation, the precise calibration of the dispensing system may be lost. Rotation of the assembly necessarily implies rotation of the needle. Unless the center of rotation is exactly in the center of the needle position, the effectiveness of prior calibrations will be compromised. This effect is exaggerated if the needle were to be bent in any way.